Inserting lines into text files can only be done by rewriting the text files. You will have to create a new file, write the three lines into it and then append monthly.txt to that file, and then rename that file to monthly.txt.

Update: See the below node by Anonymous Monk for stuff I left out and additional hints.

Some untested code : 

#/usr/bin/perl -w
use strict;

my $daily = "daily.txt";
my $monthly = "monthly.txt";

# These will hold all lines from the files
my @daily_lines;
my @monthly_lines;

# First extract the last three lines from $daily
local *DAILY;
open DAILY, "< $daily" or die "Can't open $daily: $!\n";
@daily_lines = <DAILY>;
close DAILY;

@daily_lines = @daily_lines[-3..-1];
# Now, @daily_lines contains the three last lines from the
# file named $daily

# Now we will recreate the monthly file :
local *MONTHLY;
open MONTHLY, "< $monthly" or die "Can't open $monthly : $!\n";
@monthly_lines = <MONTHLY>;
close MONTHLY;

# And write the new file :
open MONTHLY, "> $monthly.tmp";
print MONTHLY join "", @daily_lines;
print MONTHLY join "", @monthly_lines;
close MONTHLY;

# Now we delete $monthly and move our temp file over it :
unlink $monthly or die "Couldn't remove old file $monthly : $!\n";
rename( "$monthly.tmp", "$monthly" ) or die "Couldn't rename $monthly.
+tmp to $monthly : $!\n";

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In reply to Re: Reading and writing to files by Corion
in thread Reading and writing to files by damian

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